# Why do we test electric fields with positive charges and not negative ones?

Is there any difference between using a positive versus a negative charge to test an electric field?

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Are you asking about the definition "The electric field intensity is defined as the force per unit positive charge that would be experienced by a stationary point charge, or "test charge", at a given location in the field" en.wikipedia.org/wiki/Electric_field#Definition ? –  anna v Sep 13 '11 at 7:47

You can use a negative charge to test an electric field. You just have to remember that the electric field points antiparallel (opposite) to the force on the charge, rather than parallel to it (in the same direction). That's just a convention, though; we could have defined the electric field to point with the force on a negative charge, and physics would work the same, except for a couple of negative signs in some formulas.

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This is actually the answer I was looking for... –  wizlog Sep 13 '11 at 6:30
@wizlog: the system allows you to change which answer you've accepted if you think that's appropriate. –  David Z Sep 13 '11 at 7:11
opposite and parallel is not the best pairing of words, perhaps opposite and towards or away from and towards ? –  placeholder Jul 18 '14 at 4:49
@placeholder I suppose so, so I've clarified the wording, though I think it's a fairly standard convention in physics that, when talking about directed quantities like vectors, parallel means in the same direction, not just along the same axis. –  David Z Jul 18 '14 at 19:36

we take positive charge as a point charge because positive charge has a high potential and electrons are always move from lower potential to the higher potential.so , we use positive charge as a point charge

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We take positive charge as a test charge because positive charge is higher potential and negative charge is lower potential. Therefore, influence of positive charge on other charges is greater than negative charges. We can also take negative charge but the effect will be lower.

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What? This is totally wrong. The absolute potential of -1 charge and +1 charge is exactly the same (one unit of charge). –  Brandon Enright Jul 18 '14 at 3:53

You can also use the negative charges. What happens is that usually the positive charges are used because, by convention, these are the charges that they decided to use to determine the direction of the electric current. Besides, you can see it from this perspective: negative charges can move on the "lattice" and they are more "fun", while the protons are "boring", there are always "stuck" in the atomic nucleus.

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There is no fundamental difference between positive and negative charges. Anything that couples to positive electric charge couples to negative charge and vice versa. Practically, the main difference is that negative charge is on electrons and positive charge is in protons in atomic nuclei, so on a day-to-day basis they play different roles. However, these signs are reversed for antimatter, so we see there really is nothing special about the sign of a charge.

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